Mutations in the p53 tumor suppressor gene are found at a high frequency in a wide variety of primary human cancers. See, e.g., M. Hollstein et al., Science 253, 49-53 (1991). Breast cancer is typical of human carcinomas with respect to p53 involvement, and it has been reported that approximately 10% of breast cancer patients have circulating antibodies directed against the p53 protein. L. Crawford et al., Int. J. Cancer 30, 403-408 (1982). The diagnostic significance of circulating p53 antibodies has, however, been uncertain: it has subsequently been found that 21% of patients with benign mammary fibroadenomas show elevated levels of the p53 protein in tumor tissue samples, and that more than half of patients with serum p53 antibodies show no elevation of the p53 protein in tumor tissue. L. Crawford et al., Mol. Biol. Med. 2, 261-272 (1984).